Gun barrels have been made in substantially the same way since the early 1900's, with only minor improvements in processes and materials since then. Conventional steel alloys used in gun barrels, including rifles, side arms, and shotguns as well as barrels for large naval and ground artillery and high rate-of-fire weapons, such as machine guns and cannons, are heat treatable to increase their strength. However, the trade-off for attaining high strength by heat treatment in steel alloys is an increase in brittleness. Put another way, the ability of the steel alloy to yield without rupturing when its yield strength is exceeded, a property known as toughness, is reduced when the steel is heat treated to achieve high strength. A high strength brittle material in a gun barrel is dangerous because overpressure caused by a plugged barrel or excessive powder loads, or weakness in the barrel caused by damage, fatigue, corrosion, or other such factors could cause the barrel to burst catastrophically instead of just bulge. Since the bursting usually occurs at the breech end, near the shooter's face, the potential for serious injury, blinding, or death is more likely with brittle materials. Accordingly, it is the normal practice, although not universal, for gun manufacturers to sacrifice potential strength and hardness for toughness of their barrel materials by not heat treating to its maximum strength, usually less than 32 KSI for a typical high strength barrel material. As a result, the barrel wall thickness must be made commensurably thicker and the “soft” condition of the barrel material is susceptible to rapid erosion on the inside diameter of the barrel from the passage of the projectiles.
Corrosion resistance of high carbon steels is notoriously poor. Special coatings and other techniques are available to protect the gun barrels from corrosive influences such as salt water, most acids, products of propellant combustion, and many other substances common in the environment. However, such coatings are most useful if applied frequently, especially immediately after each use of the gun, but it is rarely convenient to do so. Consequently, there is a period following use of the gun before it is cleaned and coated with the protective coating during which rapid corrosion can occur, especially since the combustion products of the propellant, and the projectile fragments remaining in the barrel can create galvanic corrosion. The resultant pitting of the bore then tends to trap additional corrosive materials, further exacerbating the corrosive effects. Thus, there is a need to find barrel materials that can improve and resist the effects of these corrosive substances.
Hot plastic deformation of a conventional steel barrel is a serious problem, especially in weapon systems. At elevated temperatures, the steel barrel is effectively hot forged slightly each time the gun is fired, increasing the internal diameter of the bore slightly and, over time, increasing it enough that the bore, even without erosion, is no longer within bore tolerance. In this case, the projectile is loose in such an over-sized bore and results in poor accuracy for the gun. Moreover, the blow-by of propellant gases around the projectile in the bore is so great that the projectile does not develop the velocity it needs to attain its specified range, and instead falls short of its intended target. Thus, there is a need for a barrel material that has increased biaxial strength at elevated temperatures to eliminate the deformation of the barrel and its undesirable blow-by or blow-back effect.
A goal in designing modern military weapons is to attain higher muzzle velocity for the projectile to attain longer range, flatter trajectory, higher impact energies and greater accuracy. One conventional technique for increasing the muzzle velocity is to increase the propellant energy. The limitations of this technique are the burst strength of the barrel, primarily in the breech area when the barrel is hot. This region of the barrel is where the largest pressure spike occurs while the projectile is fired and where the primary propellant/barrel reaction occurs.
Today, guns require relatively thick-walled barrels to contain the high propellant gas pressure and provide a large heat sink to prolong the period during which high rate-of-fire can be tolerated before the accuracy deteriorates to the point beyond which further expenditure of ammunition is useless. Such conventional thick walled steel gun barrels are very heavy and have a tendency to droop at the muzzle end when aimed at low elevations. In addition, the barrel becomes hot from aggressive firing and the Young's modulus of the steel drops. This has been an intractable problem in the past because of the need for high burst strength and the high density of the only known materials that were proven for use in gun barrels. Thus, a stronger, more corrosion and wear resistant metal gun barrel that is comparatively light weight, has a high Young's modulus for stiffness, and high burst strength is needed. A gun barrel made of tough, high strength materials may be made thinner than the current barrels to reduce the weight of the barrel. In addition, the high strength and toughness of the barrel material would permit use of higher energy propellant loads for increased muzzle velocity, range and accuracy. Finally, such an ideal gun barrel would have improved wear, erosion and corrosion resistance, a low coefficient of friction with the projectile materials, a high heat capacity, and low coefficient of thermal expansion to minimize the distorting effects.
Cobalt-based superalloys are well known and widely used as liners in many steel machine gun barrels which are press-fit into the breech section of the steel barrel. The liners extend the life of the barrel by enhancing their strength, wear and corrosion resistance. For example, Stellite 21 has been in use for over half a century as a liner material for the M2 50 caliber machine gun. Typically, these liners are made to Military Specification “Cobalt-Chromium Alloy Castings” (for barrel tube liners) per Mil-C-13358E(MR) dated Jan. 4, 1984. This military specification calls for the liner to be made from a cast, cobalt alloy, such as commercial alloy Stellite 21. There are similar commercial alloys which are not cast, but rather made from a powder metal such as CCM Plus, e.g., see Table 1 below.
TABLE 1Chemistries of military specification for cobalt gun barrel liners, compared to Stelltie 21 and CCM Plus.CarbonCobaltChromiumMolybdenumNickelFeBarrel Tube Liners (Mil-C-13358F(MR)0.20~6025.5-29.54.5-6.51.75-3.252.5 (max.)Stellite 210.20-0.35~6426.0-29.04.5-6.02.0-3.03.0 (max.)CCM Plus0.20-0.30~6526.0-30.05.0-7.0
The U.S. Army is currently pursuing efforts to reduce gun barrel wear and erosion. In addition to resisting chemical attack, cobalt alloys have additional characteristics that make it attractive as a gun barrel liner. First, it is relatively inexpensive as compared to tantalum and its alloys, which are also being tested as liners. Second, cobalt alloys have sufficient shear strength high enough to resist the reaction forces of the projectile on the lands of the rifled M242 barrel. It was estimated that pure tantalum would not have a high enough strength to be used in the M242 barrel. Finally, it is expected that cobalt-based materials, such as Stellite materials, can be machined to form the lands and grooves of a rifled barrel. In contrast, difficulties have been experienced in machining an explosively-clad tantalum alloy in an M242 Bushmaster barrel. Despite the high temperature strength and wear resistant benefits that cobalt alloys have over other superalloys, cobalt liners continue to wear out from firing under hot conditions and need to be replaced over time. FIG. 1 shows a machine gun barrel that has been cut in half to show the damaged inner surface of a cobalt liner. The cobalt liners eventually fail due to fatigue from the combination of repetitive firing pulses, extreme heat and pressure and also fail due to wear from the abrasiveness of the existing projectiles. Thus, there is a need to improve the cobalt liner used in conventional barrel materials.